1. Field of the Invention
The present invention relates to a receiving apparatus.
2. Description of the Related Art
In wireless communication, a received signal is generally demodulated into a baseband signal after being subjected to a filtering process, a frequency converting process, an amplifying process, etc. Particularly, a superheterodyne receiving apparatus is known in general that mixes a received RF (Radio Frequency) signal with a local oscillator signal from LO (Local Oscillator), to be converted into an IF (Intermediate Frequency) signal. The superheterodyne receiving apparatus does not need a circuit handling high frequency wave after the frequency converting process from the RF signal to the IF signal.
The superheterodyne receiving apparatus receives not only a desired signal to be received but also an image signal having a frequency f2 (=2fL−f1) symmetrical with a frequency f1 of the desired signal with respect to a frequency fL of the local oscillator signal. Therefore, in order to prevent interference, the image signal should be eliminated when the received RF signals is converted into the IF signal.
For example, Japanese Laid-Open Patent Publication No. 2006-229619 discloses a high-frequency circuit, in which a received RF signal is mixed with each of a pair of local oscillator signals having phases orthogonal to each other and a pair of generated mixed signals are combined after the phases thereof are shifted by ±45 degrees, so that an image signal is eliminated. The high-frequency circuit mixes the RF signal and the local oscillator signals with a mixer using a Gilbert cell and the phases of the mixed signals are shifted with a polyphase filter.
For example, Japanese Laid-Open Patent Publication No. 2008-167000 discloses a receiving apparatus, in which a pair of mixed signals generated in the same manner is input to a complex BPF (Band-Pass Filter), so that an image signal is eliminated. The receiving apparatus uses a complex BPF capable of fast response, so-called Gm-C filter, includes a transconductance amplifier and a capacitance element.
As such, in a superheterodyne receiving apparatus, image signals are eliminated so that the interference can be prevented, and thus only the desired signals to be received can be received.
However, in the image signal eliminating method as described above, a phase shift and a gain deviation occurs in a pair of generated mixed signals due to variability of parts used therein and change in ambient temperature. The phase shift and the gain deviation causes the pair of the mixed signals not to have phases orthogonal to each other, or have amplitudes different from each other, the image signal remains without being completely eliminated. Thus, the interference occurs between the desired signal and the image signal, which deteriorates the communication quality of the receiving apparatus.
When a frequency of the IF signal is expressed by f0 (=|f1−fL|=|fL−f2|), a frequency difference Df between the desired signal and the image signal is as follows:Df=|f1−f2|=2|fL−f2|=2f0.Therefore, in the case of a low-IF receiving apparatus using the frequency f0 of the IF signal on the order of several ten to several hundred kHz, the frequency difference Df between the signals is small, which makes difficult to attenuate the image signals before the frequency converting process. Therefore, the image signals remarkably remain due to the phase shift and the gain deviation, particularly in the low-IF receiving apparatus.